Spindle bearing preload selection is key

A major challenge for designers of machine tool spindles with rolling element bearings is selecting and maintaining the preload in the spindle bearings. Preload means a radial load is applied to all the rolling elements, or balls, simultaneously.

Spindle bearing preload is required for static accuracy. It keeps the rolling elements in contact with the spindle and housing and prevents “play” in the spindle. A gap, or play, would allow the spindle to rattle in the housing, and the position of the spindle centerline could shift.

In addition to accuracy, preload is required for stiffness. The balls in a ball bearing assembly deform under the application of a spindle load. Their stiffness is generally less than that of the spindle body, so the balls play the role of springs. A spindle with balls contacting both sides is about twice as stiff as it is if some of the balls lose contact. The preload has to be great enough that the balls stay in contact regardless of the load on the spindle.

In addition, the balls are not linear springs. It is easy to slightly deform a ball, compressing it a little near the surface. If the ball begins to deform more, the contact patch is larger, and stiffness increases. Therefore, the balls are “stiffening” springs.

Courtesy of S. Smith

Figure 1. An angular contact ball bearing spindle in the “Big X” configuration.